The present invention relates generally to electronic relays and to techniques for controlling the temperature of electrical resistance heater elements through such electronic relays. More particularly, the present invention relates to a proportional firing circuit for electronic relays which is capable of precisely controlling the time during which electrical power is transmitted to a load, such as a resistance heater element.
Industrial plastic processing machines utilize multi-phase electricity as an energy source to generate heat for melting and/or maintaining plastic in a molten state. The electricity is converted into thermal energy by electrical resistance heating elements which are located on a molten plastic vessel associated with the molding machine. The temperature of the molten plastic is controlled by selectively turning on and off the electrical power supply to the electrical resistance heating elements. In conventional industrial plastic processing machines the electrical power is switched on and off by electromagnetic relays having mechanical contacts. Each phase of the electrical supply is switched by individual pairs of contacts.
Heat control using electromagnetic relays is capable of reliably maintaining molten plastic at a temperature range of minus zero, plus seven degrees F. Operating in such a temperature range, the relays are cycled approximately once per minute. To realize more accurate temperature control, the relays must be cycled on and off more frequently. For example, minus zero, plus one degree F. temperature regulation may be realized if the relays are cycled approximately once per second. Such high frequency relay cycling raises the operating temperature of the relay contacts and reduces the life of the contacts.
An electronic relay which overcomes the disadvantages of prior electromagnetic relays is described in U.S. Pat. No. 4,618,817. This patent issued on Oct. 21, 1986 to the present inventor and is commonly assigned. This patent is hereby incorporated by reference. The electronic relay of this patent is capable of being rapidly cycled many millions of times over its useful life. The electronic relay of this patent utilizes silicon controlled rectifier ("SCR") modules to control the transmission of electrical power from a multi-phase power supply to a heating element or other suitable load. Each SCR module includes a zero crossing detector to coordinate the firing or gating signal for the SCRs contained in the module with zero crossing junctions in the power supply.
In order to more fully utilize the capabilities of this type of electronic relay, the present invention provides a relay control circuit which is capable of reliably controlling the transmission of electrical power in very small time increments. Specifically, the present invention features a proportional firing circuit which will create an SCR firing signal whose pulse timing is proportional to the magnitude of a variable input signal. Accordingly, this proportional firing circuit will enable the conduction time of the SCRs to be proportionally varied between a minimum conduction time which is associated with the cycling of the power supply (e.g., 2.5 cycles "ON" time in the period of a second) and a maximum conduction time (e.g., full ON).
In accordance with one aspect of the present invention, the control circuit varies the conduction time for the SCRs by varying the time between firing signal pulses of fixed duration. Thus, for example, the duration of the firing signal pulse may be fixed at 2.5 cycles of a 60Hz A.C. power supply, and the "OFF" time between these pulses may be varied from approximately 1 second to 0.5 cycles before a full on state is achieved.
Importantly, the control circuit of the present invention will provide a response relative to the cycles of the power supply which is proportional to the variable characteristic of the input signal. This precise control over the transmission of electrical power will enable a suitable load, such as an electrical resistance heater element to maintain an essentially constant temperature which is desired for optimum processing of a plastic part.
Another advantage of the present invention is its ability to act as an open loop controller in the place of other controllers which cannot offer the same level of precision in its operation. The pulse timing response of the present invention also has the advantage of extending the life of the load that it is driving, because this form of operation is less stressful on the load.
In one form of the present invention, the control circuit includes a 555 timer integrated circuit, and a variable resistor circuit which provides the variable signal used to control the conduction time of the SCRs. The variable resistor circuit may be either a potentiometer or a circuit component whose resistance varies in response to an input signal. The 555 timer is provided with the appropriate timing components, such that it will generate a firing signal whose pulse cycle time is proportional to the magnitude of a variable input signal.
Additional advantages and features of the present invention will become apparent from a reading of the detailed description of the preferred embodiment which makes reference to the following set of drawings in which :